This invention pertains to a multistage, extensible-contractible hydraulic ram assembly, and more particularly to such an assembly which is especially adapted for use in conjunction with a triple-lift-type mast in a lift truck.
Many industrial lift trucks are equipped with a so-called triple-lift-type, vertically extensible-contractible mast. Such masts include inner, intermediate and outer sections which are extensible vertically relative to one another, with the inner section carrying a carriage on which any one of a variety of load-handling attachments, such as lift forks, may be mounted. The carriage is mounted for travel up and down on and along the inner mast section during what is referred to normally as the "free-lift" operating stage of a truck. Raising and lowering of the mast is accomplished through mechanism substantially always including a multisection hydraulic ram, with various sheaves and chains providing operative interconnections between the ram and the mast sections.
A number of features are usually sought in such an arrangement. To begin with, in order to avoid jolting of a load which is being handled, it is desirable that the raising and lowering speed of a load, throughout the entire vertical travel range of the load-handling attachment, be as uniform or constant as possible. It will be apparent that such jolting will occur if any abrupt changes in speed take place, and, of course, jolting presents the serious hazard of dropping a load and causing injury and damage. Further, it is desirable that the sequence in which various movable parts in and on the mast travel with respect to one another be controlled positively. In other words, for each type mast and associated carriage, there is a specific sequence of relative movements which are proper, and it is desirable to insure that nothing causes "mis-sequencing" of these parts. Further, it is well known that, because of the rough environments in which industrial trucks are often used, the more parts that are exposed on and around a mast and associated cylinder, the greater the likelihood of damage and down-time. Unfortunately, achieving of all of these desirable features in a single arrangement in the past has not been satisfactorily accomplished.
For example, efforts to produce uniform vertical travel speed for a load have resulted in compromises within ram construction which lead to mis-sequencing problems. Where rams have been constructed to produce such uniform travel speeds, sequence control has had to be performed by external latches mounted on the various mast parts, which latches produce predetermined sequenced interlocking and unlocking of the mast parts to assure positive control. Latches, of course, add considerable complexity and cost to a mast, and because of their exposed nature can easily become damaged. In masts where latching is avoided, positive sequence control has been attained through providing specially related working surface areas for pressure fluid within the associated ram--the percentage differences in the sizes of these areas positively assuring that certain parts in a ram will move before or after certain other parts. However, this approach leads, automatically, to nonuniform travel speed for a load, and thus presents the undesirable jolting problem.
A general object of the present invention is to provide a unique hydraulic ram assembly whose construction enables it to be used with a triple-lift-type mast in a manner substantially completely avoiding the problems of the past, and offering all of the features and advantages mentioned above which are considered to be desirable.
More particularly, an object of the invention is to provide such an assembly which, in and of itself, is capable of producing substantially uniform travel speed for a load, and positive sequence control over the movements of parts in a mast, without requiring any exposed external means, such as latches, for interlocking the mast sections.
A further object of the invention is to provide such a ram assembly which leads toward a relatively uncomplicated structure when combined with the mast of a lift truck-- in other words, to provide an assembly which leads to a combined structure that tends to minimize the number of exposed, potentially damagable parts.
According to a preferred embodiment of the invention, which is illustrated and described hereinbelow, the proposed ram assembly may be thought of as one including, essentially, a ram within a ram. More specifically, this assembly comprises an outer cylinder and an outer piston mounted for reciprocation on the outer cylinder. Nested within the outer cylinder, for relative extension in the opposite direction from the outer piston, are an inner cylinder and piston--the former functioning in many respects like another piston with respect to the outer cylinder. When mounted in place in conjunction with the mast of a lift truck, the assembly of the invention is disposed in what is referred to as an inverted position. This means that as the assembly extends, its larger diameter members are disposed generally above its smaller diameter members.
Special valving mechanism within the assembly is actuated by an external lever which senses when the assembly has reached a certain state of extension. This mechanism redirects the flow of pressure fluid in the assembly to set it up, so-to-speak, for the next stage of extension, whereby the travel speed of a carriage may be maintained substantially constant.
As will become apparent, fluid for extending the assembly from a fully contracted state is introduced through the exposed lower outer end of the inner piston, such fluid flowing therethrough into an elongated annular space on one side of the inner cylinder, and between the inner and outer cylinders, to act against the outer piston. Fluid flow into another space, on the other side of the inner cylinder, and also between the inner and outer cylinders (wherein fluid may act to extend the latter relative to the former), is accommodated by a special valve mechanism, which mechanism initially blocks such flow.
During extension of the assembly, what first occurs is that the outer piston alone moves, and extends relative to the outer cylinder. As the outer piston nears the end of its extension, it engages an external leverage assembly which actuates the valve mechanism mentioned above to permit fluid now to flow into the second-mentioned space between the outer and inner cylinders. At this point during the operation, fluid flows briefly simultaneously into both of the spaces mentioned. This flow condition smoothes the transition from the first stage of extension to the next stage. The outer piston continues to the completion of its extension, and at the same time the outer cylinder begins to extend relative to the inner cylinder.
The valve mechanism referred to above takes the form of a slidable, hollow, tubular spool, in which there is provided a central spring-biased closure device that initially blocks flow through the spool to prevent movement of the outer cylinder. It is this closure device which is later unseated as the outer piston nears the end of its extension. As the outer cylinder begins to extend relative to the inner cylinder, the spool in this mechanism shifts, eventually, relative to the inner cylinder in the same direction as the outer cylinder, and blocks a port which it previously held open that allowed fluid to flow to act on the outer piston. With such blockage, fluid acting against the outer piston becomes captured.
With continued extension of the outer cylinder, the annular space between it and the inner cylinder contracts-- requiring that fluid be exhausted from this space. Such exhausting fluid is now directed through a port in the inner cylinder to act against the inner piston. As a consequence of this capturing and fluid-directing operation, the inner cylinder extends automatically relative to the inner piston.
The effective working surface areas of the various movable parts in the assembly are selected preferably to produce and extension characteristic for the assembly whereby it is capable of producing constant travel speed in a carriage on a mast. The specific relationships of such areas which provide this characteristic will be explained more fully below.
Incorporation of valving mechanism, like that just generally outlined, within the confines of the ram assembly, eliminates, as it will be seen, the necessity for using external latches. It should be noted that the valving mechanism which obviates the need for latches is itself well protected within the confines of the overall ram assembly.
These and other features and advantages which are offered by the invention will become more fully apparent as the description which now follows is read in conjunction with the accompanying drawings.